Carrier fluids under high pressure, such as under supercritical conditions, are increasingly employed in wood treatment processes for impregnative or extractive purposes. Fluids under high pressure have properties partly similar to both gases and liquids. Thus the penetration properties of supercritical fluids are similar to gases whereas the solubilisation properties are similar to liquids.
Carbon dioxide is a very attractive compound for use as a supercritical medium for treatment of wood because of a suitable critical point (31° C., 73 bar), a low chemical reactivity and a low toxicity. Further carbon dioxide is available in large quantities at a relative low cost.
An article of Morrell and Levien: “Development of New Treatment Processes for Wood Protection” Conference report from “Conference on Wood Preservation in the '90s and Beyond” Savannah, Ga., USA, Sep. 26-28, 1994, deals with impregnation of wood species normally resistant to impregnation, by using supercritical carbon dioxide to deliver and deposit biocide into said wood. In the paper experiments are described where wood samples of 100 mm or less are used for the impregnation.
U.S. Pat. No. 5,364,475 describes a process for removing chemical preservatives by extraction using supercritical carbon dioxide as the extractive medium and wood samples of a size of 10×50 mm.
WO 00/27601 discloses an impregnation of wood using supercritical carbon dioxide where the pressure is released after the treatment in a pulsating way in order to avoid or reduce resin exudation to the surface of the wood.
In the literature there have been reports of change of mechanical properties of wood samples treated under supercritical conditions.
In Anderson et. al. 2000, Forest Products Journal, 50:85-93, it is described that the mechanical properties are affected by the supercritical treatment. Western red cedar showed a reduction of up to 23.1% of modulus of rupture and up to 13.7% reduction in modulus of elasticity. Further it is described that some samples exhibited dramatic treatment defects and were spilt into hundreds of long slender sticks. It was further described that a relationship exists between sample size and damages to the wood, where larger samples are more damaged that smaller samples. The observed damages were allegedly caused by pressure gradients inside the wood.